1. Field of the Invention
The present invention relates to a liquid crystal display element using a cholesteric liquid crystal.
2. Description of the Related Art
Recently, the development of electronic paper has been advancing vigorously in business organizations and universities. For the electronic paper application markets that are expected, proposed are various application forms including electronic books heading the list, sub-displays for mobile terminals, display parts for IC cards, etc.
One of the most promising modes for the electronic paper is one using a cholesteric liquid crystal. Cholesteric liquid crystals have excellent characteristics such as semi-permanent display maintaining function (memory function), bright color display, high contrast, and high resolution.
Cholesteric liquid crystals are sometimes called chiral nematic liquid crystals. A cholesteric liquid crystal can be obtained by adding a relatively large amount (several tens of percentage by weight, for example) of a chiral additive (also called chiral agent) into a nematic liquid crystal, to put the nematic crystal molecules in a strongly spirally wound state (which is also called a cholesteric phase), so that the incident light is reflected and interfered.
The principle of display and driving of a liquid crystal display element using a cholesteric liquid crystal is shown below. Display with a cholesteric liquid crystal is controlled by means of the alignment states of the liquid crystal molecules. In the alignment states of a cholesteric liquid crystal, there are a planar state 2 which reflects incident light 1, and a focal conic state 3 which transmits the incident light 1 as shown in FIG. 1. They are present stably under no electric field application. Reflected light 4 enters a human eye 5.
In the planar state, light having a wavelength which corresponds to the spiral pitch of the liquid crystal molecule is reflected. Wavelength λ for which the reflection is the greatest is shown in the following formula with the average refractive index n and the spiral pitch p of the liquid crystal. The reflection hand Δλ which is the peak width value at the half height of the reflectance peak when the wave length is plotted as abscissa, and the light reflectance is plotted as ordinate, becomes larger with the increase of the refractive index anisotropy Δn of a liquid crystal. That is, the larger Δn is, the brighter the display is.λ=n·p 
To compare, in the focal conic state, light passes through the cholesteric liquid crystal. Accordingly, it is possible to prevent light reflection by installing a light absorbing layer at the back of the liquid crystal layer, so as to display black color.
FIG. 2-A, B are examples of driving of a liquid crystal display element using a cholesteric liquid crystal. When a strong electric field is applied to a cholesteric liquid crystal, the spiral structure of the liquid crystal molecules becomes completely loose to be in a homeotropic state in which all the molecules follow the direction of the electric field. In this operation, alternating current is applied as shown by numeral 21 in FIG. 2A, so as to prevent the degradation of the liquid crystal. These voltages are the driving voltages to obtain a planar state. When the electric potentials are changed into zero sharply from the homeotropic state (indicated by the numeral 22 in the case of FIG. 2A), the axis of the spiral of the liquid crystal is made perpendicular to the electrode, and the planar state is formed which selectively reflects light, corresponding to the spiral pitch. Here, it is to be noted that although a rectangular shape wave is used in all the cases in this figure, the wave shape is not limited to this, and various shapes may be used in combination.
On the other hand, when voltages (shown by numeral 23 in FIG. 2B) that are so weak that the spiral structure of the liquid crystal molecules of a cholesteric liquid crystal do not become loose are applied (these voltages are the driving voltages for obtaining a focal conic state), followed by the removal of the voltages (shown by numeral 24 in FIG. 2B), or when large voltages are applied, followed by gradual voltage decreasing, the axis of the spiral of the liquid crystal becomes in parallel with the electrodes, forming the focal conic state to transmit incident light. Furthermore, when voltages of a medium level are applied, followed by abrupt removal, there is a mixture of the planar state and focal conic state, making it possible to display an intermediate tone. With a cholesteric liquid crystal, information display is carried out, utilizing these phenomena.
It is known that the response speed of a liquid crystal becomes the larger, the smaller the viscosity of the liquid crystal is, or the smaller the liquid crystal thickness (or cell gap) is. Also, in the case of a cholesteric liquid crystal, the smaller the viscosity is, the quicker the response to a homeotropic state is, when voltages are applied. That is, as the viscosity is made smaller, a planar state is made possible with a shorter pulse width (pulse application time), resulting in decrease of the driving voltages.
Conversely, when the viscosity is larger, the time to get to a homeotropic state becomes longer. Therefore, it is necessary to apply a voltage for a longer time, or apply a higher voltage, in order to achieve a planar state. When the viscosity becomes even higher, a full focal conic state becomes hard to achieve, decreasing the transmittance and increasing scattering of light.
Due to such circumstances, desired contrast is not provided sometimes when a liquid crystal display element is driven at a low temperature. While the viscosity of a liquid crystal composition generally rises as the temperature decreases, it is also important from the viewpoints of driving and display quality to suppress the viscosity rise to as small a extent as possible.
FIG. 3 shows a voltage response characteristic in a solid line during the transition from an initial planar state to a focal conic state. The abscissa axis represents the absolute value of the driving voltages in a set of operations to apply driving voltages, and then stop the application (such as those represented by numerals 21, 23 in FIG. 2-A, B). For example, when the driving voltages are a combination of +32V and −32V, the absolute value is 32V. In the following, the absolute value of driving voltages is sometimes simply called a driving voltage. The liquid crystal reflects light in an initial planar state zone 31. After that, when the absolute value of the pulse voltage (that is, driving voltage) is raised from 0 V, it passes through a transition state zone 32 into a focal conic state zone 33, and when the value is further raised, through a transition state zone 34 into the planar state zone 31.
In contrast, when the liquid crystal changes from the initial focal conic state to the planar state, as shown in a dotted line of FIG. 3, the liquid crystal transmits light in the initial focal conic state zone 35. After that, when the absolute value of the pulse voltage is raised, it passes through a transition state zone 36 into the planar state zone 31.
It is to be noted that while a cholesteric liquid crystals provides a certain level of brightness if its thickness (or cell gap) is 2 μl or larger, the brightness in the planar state is improved as the thickness is increased. For this reason, thicker cell gaps are generally preferred.
Liquid crystal display elements using cholesteric liquid crystals have some problems in practice as follows.
(1) The Driving Voltage of a Liquid Crystal Display Element
The driving voltages of a liquid crystal display element may be divided into a driving voltage to put the liquid crystal in a planar state, a driving voltage to put it in a focal conic state, and a driving voltage to put it in a transition state. However, when the term is used without limitation, they refer to the largest among them, that is, the driving voltage to put the liquid crystal in a planar state.
Modes using a cholesteric liquid crystal employ a higher driving voltage than other modes such as the electrophoresis method for use in electronic paper, that is, in the range of 35 to 60 V, in general. On this account, costs for parts for the driving circuit are rather high. In order to overcome the cost problem with a general-purpose IC for an LCD driver for liquid crystal display elements, a driving voltage of 35 V or below is indispensable, and if a margin is considered, it is important to decrease the driving voltage as low as 32 V or below.
(2) Electric Power Consumption
As RF-ID (Radio-Frequency-Identification: a contact-free individual identification technology to perform identification/communication using electric waves) has been becoming popular rapidly in recent year, demand for a display device in which a liquid crystal display element can be driven via RF (radio waves) is also increasing. In order to apply a cholesteric liquid crystal to this RF-ID, it is important to decrease the electric power consumption, in addition to the above-described decrease of the driving voltage. For this purpose, adjusting the dielectric constant to decrease the electric capacitance to an appropriate level is also necessary, in addition to the prevention of leak current to the utmost through a larger liquid crystal resistance.
(3) Display Maintaining Temperature
Display maintaining temperature means a temperature in which a liquid crystal can maintain the planar state or focal conic state even after a long time period of standing.
In the present level, the planar state or focal conic state can be maintained at least after several days at a temperature generally in the range of about −10 to 60° C., and further expansion of the display maintaining temperature is being desired. For example, a display maintaining temperature in the range as wide as about −20 to 90° C. is being desired for parts for vehicle uses.